J. Phys. Chem. 1991, 95,9075-9080
higher temperatures would be ascribed to population of a nonemitting state close in energy to the two BIstates identified earlier. The presence of the butyl substituents, which would displace solvent molecules, could result in a less polar local environment and a unique excited-state ordering for this complex.2s On the other hand, the dbp complex is also unique in that it has floppy substituents in the 2- and 9-positions of the phenanthroline. It is therefore possible that the emission is influenced by some type of thermally activated quenching process that is peculiar to the complex with the butyl groups.
Conclusions Excitation and emission polarization data provide a means of assigning the absorbing and emitting charge-transfer excited states of C U ( N N ) ~ systems. + Although the absorption spectrum can for the most part be treated within the context of Du symmetry, a static or dynamic flattening distortion is invoked to account for a low-energy shoulder designated band I. In line with theory, the dominant absorption (band 11) has been assigned to a z-polarized 'B2 term, where z is the axis connecting the metal and the ligand centers. A high-energy shoulder (band HI),which is absent in the bipyridine complex, has been shown to be x y polarized and is attributed to a 'E state involving excitation to the ligand SLUMO. At least two emissive states in thermal communication are required to explain the temperature dependence of the emission intensity, and the emission polarization data reveal that both
9075
emissions are z polarized. Consequently, they cannot be assigned to spin-orbit components of a 'E state as proposed by Parker and Crosby.8 The polarization data can, however, be reconciled with the model of Kirchhoff et al., which assigns the emission to singlet and triplet CT states." However, the complex is believed to undergo a significant flattening distortion in the C T excited state, and in the reduced symmetry the z-polarized emissions have to be assigned to singlet and triplet terms associated with different electronic configurations. In the absence of more specific information about structure, the emission assignments have been couched in terms of D2 symmetry which is believed to be the highest possible symmetry for the C T states. However, it is possible that this assumption will be inadequate for explaining results obtained with other techniques. In fact, resonance Raman data have been interpreted to indicate that the electron localizes on a single N N ligand in the excited state,26in which case symmetry could be as low as C,. The uncertainties regarding the molecular symmetry in no way affect the conclusion that the thermally activated emission is associated with an excited state with singlet multiplicity. Acknowledgment. This research was funded by N S F Grant CH-9024275. (25) For a related effect see: Reitz, G.A.; Demas, J. N.; DeGraff, B. A.; Stephens, E. M. J . Am. Chem. S a . 1988, 110, 5051-5059. (26) McGarvey, J. J.; Bell, S.E. J.; Gordon, K. C. fnorg. Chem. 1988.27, 4003-4006.
Conformational Changes on Electronic Excitation of the Mercury-Water van der Waals Complex Marie Christine Duval and Benoit Seep* Laboratoire de Photophysique Mol5culaire du CNRS, Bcitiment 21 3, and Institut de Physicochimie Mol5culaire de I'Universit5 Paris Sud, UniuersitC de Paris Sud, 91405 Orsay, France (Received: April 2, 1991; In Final Form: June 21, 1991)
-
The van der Waals complex of mercur with water has been characterized in a supersonic expansion by optical absorption in a region close to the mercury 'PI YSOatomic transition. After excitation the complex dissociates into metastable mercury (3P0)and water, and this appears to be the only open channel. The rotational contours of the bands of a Hg-H20 state assigned to a parallel transition reveal a drastic reduction by 9 wavenumbers of the A rotational constant. This reduction has been explained as being due to a change in geometry from a floppy ground state to a near-C, excited complex.
The van der Waals binding of rare gas atoms to metals' has been quite extensively studied; however, there are much fewer examples for molecules attached to metals.*-' (1) (a) Smalley, R. E.; Auerbach, D. A.; Fitch, P. S.;Levy, D. H.; Wharton, L. J . Chem. Phys. 1977.66, 3778. (b) Tellinghuisen, J.; Ragone, A.; Kim, M.S.;Auerbach, D. J.; Smalley, R. E.; Wharton, L.; Levy, D. H. J . Chem. Phys. 1979, 71. 1283. (c) Zanger, E.; Schmatloch, V.; Zimmermann, D. J . Chem. Phys. 1988, 88, 5396. (d) Funk, D. J.; Kvaran, A.; Breckenridge, W. H.J . Chem. Phys. 1989, 90,2915. (e) Kowalski, A.; Funk, D. J.; Breckenridge, W. H. Chem. Phys. Lctr. 1986,132, 263. (f) Bennett, R. R.; Mc Caffrey, J. G.;Breckenridge, W. H. J . Chem. Phys. 1990,92,2740. (g) Tsuchizawa, T.; Yamanouchi, K.; Tsuchiya, S . J . Chem. Phys. 1988,89, 4646. (h) Callender, C. L.; Mitchell, S.A.; Hackett, P. A. J . Chem. Phys. 1989. 90, 2535. (i) Callender. C. L.; Mitchell, S. A.; Hackett, P.A. J. Chem. Phys. 1989, 90, 5252. Q) Dedonder-Lardeux, C.; Jouvet, C.; Richard-Viard, M.;Solgadi, D. J. Chem. Phys. 1990, 91, 2828. (k) Cheng, P. Y.;Willey, K. F.; Duncan, M. A. Chem. Phys. Left. 1989, 163, 469. (I) Jouvet, C.; Lardeux-Dedonder, C.; Martenchard, S.;Solgadi, D. J . Chem. Phys., in press. (2) (a) Fuke, K.; Saito, T.; Kaya, K. J . Chem. Phys. 1984,81, 2951. (b) Fuke, K.; Saito, 7.;Nonose, S.;Kaya, K.J . Chem. Phys. 1987,86,4745. (c) Duval, M. C.; Jouvet, C.; Soep, 8. Chem. Phys. Lett. 1985, 119, 317. (d) Castelman, A. W., Jr.; Keese, R. G. Science 1988, 36, 241. (e) Lessen, D. E.; Asher, R. L.; Brucat, P. J. J . Chem. Phys. 1990, 93, 6102. (3) (a) Y:manouchi, K.; Isogai, S.;Tsuchiya, S.;Duval, M.C.; Jouvet, Soep, B. J . Chem. Phys. 1988,89, 2975. (b) Duval, C.; Benoist d Azy, 0.; M. C.; Soep, B. Chem. Phys. Lett. 1987, 141, 225.
0022-3654/9 1/2095-9075$02.50/0
Mercury in its gound electronic state forms van der Waals complexes which are ~ e a k l ybound ~ * ~ owing to the 6s2 closed shell configuration of the metal atom, but conversely, the excited states of 6s6p open shell character are deeply bound or reactive."Js This change of the binding energy upon excitation can be drastic-from 100 cm-l in the ground state to 6000 cm-' in the excited state, discussed in a forthcoming paper6-and may be related to chemisorption at metal surfaces. The connection, amply discussed in theoretical results from the possible similarity of the hybrid spd character of metal surface electrons to the 6p character of excited mercury. As an extension of our previous studies on the mercury-ammonia complex, we present here results on the related mercurywater complex. In the Hg-NH3 complex4 the ammonia lone pair points toward the mercury atom and there is no noticeable change in conformation between ground and excited (63PI) states. (4) Jouvet, C.; Soep, B. Chem. Phys. Lefr. 1983, 96,426. Breckenridge, W. H.; Jouvet, C.; Soep, B. J . Chem. Phys. 1986,84, 1443. ( 5 ) Callear, A. B. Chem. Rev. 1987, 87, 335. (6) Breckenridge, W. H.; Duval, M. C.; Soep, B. To be published. (7) Saillard, J. Y.;Hoffmann, R. J . Am. Chem. Soc. 1984, 106, 2006. (8) Garcia-Prieto, J.; Ruiz, M. E.;Novaro, 0. J . Am. Chem. Soc. 1985, 107. 7512.
0 199 1 American Chemical Society
9076 The Journal of Physical Chemistry, Vol. 95, No. 23, I991
Duval and Soep
TABLE I: Constants in cm-I m d A w,(stretch) (40) 173 f 5
X state A state O ( B
+ C)/2
= 0.07653 for Hg,,-H20 in ref 21.
%&
DO
B,b
Re, A
(1) 2.40 f 0.2
300 f 30 2750 f 200
0.0800 0.176 f 0.004
3.57 2.4 f 0.05
ba’, = (4.6 f 0.5) X
However, despite a I-A shortening of the intermolecular bond upon excitation, none of the N H bonds is weakened nor is the pyramidal H-N-H bond angle appreciably changed. Consequently, there is no sign of reactivity within the complex. The excitation results in a 20-fold increase in the strength of the Hg-NH3 bond, and this relates to the formation of the well-known excimer observed in H B ( ~ P , ) N H 3 c o l l i s i ~ n s . ~ ~ ’ ~ Here we show that mercury-water complexes also form strong bonds with the excited (’PI) state, but only half so strong as those with ammonia. On the grounds of well-resolved rotational contours we have established that the water complex undergoes a major conformational change upon excitation, going from quasi-free H 2 0 rotation in the ground state to a C, structure in the excited state. The decay channels are also different from the ammonia complex where fluorescence emission competed with dissociation to form the lower 3P0multiplet. In the water complex, the intramultiplet decay is the only observed decay channel.
IO->.
+
4-, A STATE
Experimental Section The setup is similar to the one used in previous experiments on mercury complexe~.~ Briefly, mercury vapor is seeded in a continuous free jet of argon at a backing pressure of an atmosphere. The argon entrains water vapor from a capsule at room temperature. The resulting average rotational temperature of the molecules is 4.5 K. The water complex is excited by a frequency-doubled (lithium formate) tunable dye laser pumped by the third harmonic of a Yag laser. Long scans, Le., 1500 cm-I, were taken with coumarin 500 dye with a 0.5-cm-I resolution (in the harmonic) for vibrationally resolved spectra, while for the 20-cm-’ rotational scans, a resolution of 0.1 cm-’ was achieved. Fluorescence of the complex was never observed. Instead, we probed for the dissociation products of the complex with a second, probe laser, searching for HgH and Hg(3Po);only the latter species was detected. Here we present action spectra, where the excitation laser scans the mercury water absorption spectrum while a second laser detects the product Hg(’P0) by the Hg(63Po-73Sl) transition at 4047 A.
Results The Hg-H20 complex has been excited optically in regions close to the Hg(’So-3PI) atomic mercury transition. We have investigated the possible relaxation pathways of the excited complex: Hg-H2O(’PI) Hg-H20 + hu, fluorescence (1)
-
-
Hg-H20(3P,) Hg-H20(3PI) Hg-H20(’PI) Hg-H,O(’P,) Hg-H20(’P1)
-
relaxation
HgH
-
hu
(2)
+ OH,
product formation, AE = 1180 cm-’ (3)
+ H,
very exothermic, AE = 13 000 cm-’ (4)
HgOH
-
Hg-H20* excimer
Hg(lSo) + O H
closed, AE = -1850 cm-’ (5)
+ H20,
intramultiplet relaxation (6) Hg-H20(’PI) Hg(’S0) + H 2 0 (vibrationally excited), radiationless intercombination to the ground state (7)
-
Hg(’P0)
+ H,
(9)(a) lsogai, S.;Yamanouchi, K.; Okunishi, M.; Tsuchiya, S.J . Chem. Phys. 1988.88.205. (b) Herzberg. G. Spectra of Diatomic Molecules; Van Nostrand Reinhold: Princeton, NJ, 1950; p 141. (IO) Callear, A. B.; Freeman, C. G.Chem. Phys. Leu. 1977945, 204.
cm-1
Figure 1. Action spectra detecting metastable mercury. (a) Mercuryammonia. (b) Mercury-water.
The only detected channel is the intramultiplet relaxation (6). Despite extensive searches no emission of light was observed, except from the mercury dimer whose absor tion lies in the reddest part of the observed spectra at -2650 . Thus channels 1 and 2, corresponding to resonance and excimer emission, are closed. The excimer emission observed in collisions of excited 3P mercury and H20I2or NH310*’1 results from the stabilized levels of the excited Hg-H20 complex and should not appear here, in collision-free conditions. No evidence of HgH formation through reaction 3 has been found. On the other hand, HgOH formation (channel 4), although energetically allowed, may have a high barrier to formation through insertion of mercury into the H-OH bond. The spectroscopic identification of HgOH has not yet been reported, but the transitions and constants should be similar to those of the isoelectronic HgF (formation energy = 1.8 eV).gb The 7th channel could not be directly monitored, and there is no conclusive evidence of its role. To investigate the intramultiplet relaxation to Hg(3Po), we have recorded action spectra where a probe laser monitors metastable 3P0 mercury at 4047 A while a tunable pump laser scans the Hg-H20 absorption, in vibrationally or rotationally resolved spectra. Vibrational 3P0Action Spectra. A typical spectrum is displayed in Figure l b extending from the forbidden ’So-’Po mercury transition through the ‘So-3PIone. Two domains, A and B, appear distinctly and correspond to the Franck-Condon envelopes of at least two distinct progressions, A being the reddest. The A progression drops suddenly in intensity at the red edge of the spectrum where the energy provided by the exciting photon becomes insufficient to dissociate the excited complex into Hg(’P0) + H 2 0 (channel 6 ) . Therefore, this breakoff measured for HgH 2 0 and Hg-D20 provides a direct measurement of the dissociation energy of th%ground-state complex: D’b = 330 f 30 cm-’ (see Table I). This action spectrum is compared in Figure 1 to the equivalent spectrum for Hg-NH3 (Figure la). We notice the existence of the same two domains in both spectra with long progressions of
8:
( I I ) Hirogushi, H.; Tsuchiya, S.Chem. Phys. 1986, 108, 153. 70, (12) 1761. Callear, A. B.; Connor, J. H. J . Chem. Soc., Faraday Trans. 1974,
The Journal of Physical Chemistry, Vol. 95, No. 23, I991 9077
Mercury-Water van der Waals Complex
A STAT€
ffgffpo
_1
I6
.
I4
1
1
I I
T
IO 1
1
1
A STAE
I
-
%
j#g-D20
%t
I2 1
ACTION SPECTRUM
..
150
-1000
I
I
I
I
6 v'rl 20 Figure 2. Birge-Sponer plot of the stretching bands in the A state of Hg-H20. (a) Hg-H,O two data sets are represented in the plot. (b) Hg-D20. IO
ca. 150 wavenumbers in the A region, but the B spectrum of Hg-H20 in Figure 1 b is of complex structure as opposed to the sharp Hg-NH, B bands. The band spacing in the A domain indicates a deep potential well, and the linear Birge-Sponer plot represented in Figure 2 shows that the upper-state potential can be represented for this vibration by a Morse oscillator. We assign this A progression to the Hg-H20 stretching mode by comparison with Hg-NH3. Also, the anharmonicity deduced from the Birge-Sponer plot scales as the inverse of the complex reduced mass when water is substituted with deuterated water, as shown in Figure 2:
Au (cm-1) Figure 3. Action spectra, detecting 3Po,of (a) Hg-H20 and (b) Hg-DzO. The arrows denote, in the A domain of the top view, between us = 13 and 16, the triple heads of the B transitions. H, FREE ROTOR LIMIT
- yo
COMPLEX
t i I NDEREO ROTOR
RIGID ROTOR
f
EXC I TED STATE
K' =1 v =o KB=O
while (9)
These values are unfortunately somewhat inaccurate due to the perturbations in the Hg-D,O spectrum, as seen in Figures 2b and 3b, but they leave little ambiguity about the stretching nature of the A bands. Rotational Contours of the A Bands. The vibrational spectrum in Figure 1 b shows double-headed bands with different relative intensities for the hydrogenated (Figure 3a) versus the deuterated complex (Figure 3b). In Hg-H20 the low-frequency head is -3 times more intense than the high-frequency head. A reversal of the band head intensities is observed for the deuterated complex, with a low/high frequency intensity ratio of -0.5. The observation of only two rotational subbands with these typical intensity ratios has led us to assign these A bands as parallel transitions of a rigid prolate symmetric top with hydrogen and deuterium nuclear spin statistics in C , symmetry and with a reduction of the A rotational constant in the excited state. The energy diagram is represented in Figure 4, taking into account two levels will be populated corresponding that, in C , ~ymmetry.2~ to ortho and para states, even at 0 K; therefore, in a parallel transition, only two bands will appear. This arises from the
OROCH)
STATE
000
I' K:O
K"=l v,=o
K" =O v,=o
Figure 4. Schematic level diagram representing the evolution from a free H 2 0 rotor to a rigid C, complex. The lowest populated levels of H20 are the Om and lo, levels, owing to nuclear spin statistics, while in the C, rigid complex these levels correlate to K" = 0 and 1 of the lowest level with the same ortho/para symmetry. The optical, parallel A-type tran-
sitions are represented. absence of relaxation of the nuclear spin in supersonic expansions, leaving the lowest ortho and para rotational levels populated. It should be noted that the relevant observed K subbands correlate, in the quasi diatomic limit, to a O+-O+ transition in Hund's case c. The change in the A rotational constant between ground and excited states sets the spacing between the two subbands, as can be seen in Figure 5, K'- K" = 1 being the reddest, from the above considerations. We have fitted the observed contours represented in Figure 5 with a model developed by Yamanouchi et a1.,9v38for Hg-NZ,
Duval and Soep
9078 The Journal of Physical Chemistry, Vol. 95, No. 23, 1991
has allowed a good simulation of the spectra of the aforementioned Hg-N2 complex3 and gives an excellent reproduction of the Hg-H20 contours, as seen in Figure 5. From the fit we can draw the following conclusions. The mercury isotope effect allows the determination of the Hg-H20 stretching potential, assuming that a Morse oscillator describes this movement. In such a case the vibrational shift is represented by:9b hu,,di,i+l) = (1
-
p)we(n,
+ !/J - (1 - PZ)weXe(ns+ ’/2)* (10)
where we and w,xe are the frequency and anharmonicity of the stretch, n, is the quantum number, and p is the ratio of the square root of the reduced masses of the ith and ( i + 1)th isotopes: P =
Figure 5. Rotational contour of the 0-1 I transition in the A spectrum (Hg-H20) (upper, experimental; lower, calculated with the values in Table 11) for a rigid rotor and a convolution width of 0.1 cm-I.
,
Isotopic shift
1
05/;:;::
::;
“stretch
Figure 6. Total isotopic shifts Au for isotopes 202 and 204 of mercury as deduced from the rotational contours such as in Figure 5 . Two fits are given, and tit b is chosen for the vibrational assignment of the stretch mode.
where the complex is assumed to be a rigid rotor. The simulated spectra are the sum of individual spectra of each mercury natural isotope shifted by Au, = AvVlb Also. The first term is the mercury mass effect in the nth stretching vibrational quantum, and the second is the isotope electronic shift that we consider as unchanged in the complex from the mercury atom. We have assumed a Boltzmann population distribution with a temperature of 4.5 K for the rotational levels. No deviation to this distribution was apparent at higher J or K . The calculated spectra have been convoluted with a 0. I-cm-’ Lorentzian line shape which seems essentially experimental and sets a lower limit of 50 ps to the dissociation rate of the complex to Hg(3Po) + H 2 0 . This model
+
[II(H~,-H~O)/~(H~~+I-H~~)I’”
This has resulted in the assignment of the vibrational stretching quanta of the A bands as shown by the fit in Figure 6 of the vibrational shift to the values displayed in Table I. We thus could determine a well depth for the A state, Do = 2750 cm-’. The overall shapes of the contours and the band heads designated by arrows in Figure 5 (isotopes 202 and 204) are very sensitive to the isotopic shift. This leads to a precision of f l in the vibrational assignment within the Morse potential approximation. The rotational structure corresponding to the ortho and para subbands is well reproduced, implying that the complex either has the C , geometry of water or leaves the water entity close to free rotation. The spectra fitted as belonging to a rigid prolate symmetric top show a 2-fold geometry change upon excitation: (1) a reduction of the Hg-H,O intermolecular distance as deduced from the increase of the B rotational constant in the excited state; (2) a conformational change, which we infer from the surprising decrease of the A constant in the excited state. The Hg-H,O bond length decreases by 21 A upon excitation, from the change of the B constant corresponding to the overall rotation, as seen in Table I. In the rotational contour fits the determination of B’- B”is more accurate than that of the absolute values of either B’or E”. There is a ensemble of B’and B”values which fit the spectra. We chose the equilibrium distances which allow the best Franck-Condon simulation of the intensities of the vibrational transitions, as shown in Figure 7. This choice sets R b - R’b = 1.05 f 0.05 A, while B ‘ - B”yields l/R’:-I/R’‘:, and we thus obtain the set of R‘” as listed in Table I. The A rotational constant is smaller by factor of 2 in the excited state (from the ortho, para subbands assignment). This rotational constant refers to the rotation of the lighter H atoms about the Hg-H20 axis in the rigid rotor limit. This decrease of the A constant arises from a lengthening of the O H bonds in the excited complex or from a Hg-H,O conformational change (which we favor as an explanation). Librational Motion. Within the A domain in the vibrational spectrum of Hg-D,O, one observes extensive vibrational structure
nr
17
I6
15
14
13
I2
11
IO
0
W’S
6
7
10
17
16
15
14
13
I? 1 1
10
0
8
V*a
Figure 7. Franck-Condon simulations in a quasi diatomic approximation of Hg-H20 (left) and Hg-D,O (right). AR was set for both simulations at 1.05 A and the vibrational constants as in Table 1.
The Journal of Physical Chemistry, Vol. 95, No. 23, 1991 9019
Mercury-Water van der Waals Complex in addition to the stretch bands, due to the other two van der Waals modes (bending modes). These bands do belong to Hg-D20 since we were, of course, very careful to eliminate normal water contamination, as is Seen from the absence of the main water A bands in the deuterated spectrum. We searched for a repeating pattern of bands built upon the stretch progression, and we propose in Figure 3 a possible structure which is best exemplified for the 0: = IO band. However, this pattern smears out in the region of ub = 14,in the region where the stretching bands become heavily perturbed in position as seen in the Birge-Sponer plot (Figure 4). These perturbations probably arise from perturbations due to the dense manifold of bend-stretch combination levels, such that beyond the o: = 14 level the bend-stretch mode separation becomes ambiguous in Hg-D20. On the other hand, the upper spectrum of Figure 3 displays only a stretch progression and a few weak bands that we shall connect to the B state in the following. We have also observed only one strong local perturbation in the Hg-H20 spectrum for the u: = 12 band, which also presents a satellite. This agrees with a much smaller density of bending levels for the Hg-H,O A state, owing to its higher bending frequency as compared to Hg-D20, in the hindered rotor description. The B Spectral Region. In Figure 1 there is a second envelope to the blue of the A domain but with a dense array of bands. This series of bands starts in the A spectrum with well-separated bands with a different rotational contour, which appear triple headed as shown by the arrows in Figure 3a (between u'= 14 and 15 and between 15 and 16). This triple head contour is the signature for the electronic nature of the transition X B which should be, in the quasi diatomic limit, similar to an SZ = 0 SZ = 1 transition. The observation of this X B transition, identified in analogy with the equivalent transition in Hg-NH315, results from the lifting of the degeneracy of 3PI mercury within the complex. The bands become quickly too dense for an analysis, but from the separation of the first observed bands (r150 cm-I) one can infer that the A and B wells are of comparable depth.
-
-
-+
Discussion Decay Channels. The only decay channel observed after the excitation of the complex is the dissociation into H S ( ~ P + ~ )H 2 0 . That no resonance fluorescence is observed proves the existence of a rapid predissociation to this competing channel (6). On the other hand, one should detect by increasing the backing pressure, the emission arising from higher order clusters, Hg-(H20),,>I. This "excimer" emission, as for the ammonia clusters Hg(NH3)*n>1,15 should result from a fragmentation onto the vibrationless level of Hg-(NH3)*,, + NH,. The excimer emission, while strong for the Hg(3Pl) + NH3'OJ' collisions, is found to be very weak for H&P,) + H 2 0 collisions.'2 The explanation may be found in the shallower well depth of the water clusters, whose bond strength is less. Nevertheless, the bottom of the A well in the water complex is likely to be the emitting state of the Hg-H20 excimer as shown in Figure 8b, where we find a striking similarity between the simulated A-X Franck-Condon emission (from the u' = 0 level of the upper state) and the observed excimer fluorescence, from coIIisions.12 The absence of reactivity of the excited water complex was not a total surprise to us, as (1)the decay through channel 6 is very efficient and (2) the HgH formation in collisions of 'P mercury and water has been very much d i ~ p u t e d ' ~and , ' ~ may have a high activation threshold not accessible a t all through the excitation of the complex. In addition, the formation of the strongly bound HgOH should be a preferred reaction channel through the same insertion Harker, A. B.; Burton, C. S. J . Chem. Phys. 1975, 63, 885. Callear, A. B. Chem. Rev. 1987, 87, 337. Duval, M. C.; Soep, B.; Van Zee, R.; Bosma, W.; Zwier, T. J. Chem. Phys. 1988,88. 2148. (16) Hutson, J . M. J . Chem. Phys. 1990, 92, 157. (17) Cohen. R. C.; Busarov, K. L.;Laughlin, K. B.; Blake, G. A.; Havenith, M.; Lee, Y . T.; Saykally, R. J. J . Chem. Phys. 1988, 89, 4494.
& cak.
2600A
32 00
I
I
Figure 8. Bound free emission from the (a) Hg-NH, and (b) Hg-H,O spectra, taken from Callear et aLi0*'*and bound free quasi diatomic Franck-Condon calculations with the parameters of ref 15 (Hg-NH,) and of this paper for Hg-H20.
mechanism, in terms of energetics, but it could have an even higher barrier. We scanned, in pump probe experiments, the probe laser in the 4000-Aregion where HgOH should absorb, in analogy with the mercury halides (X B transitions), without detecting this product. We therefore infer that channel 6 dominates over the others. We conclude, in agreement with a model relating the Hg-H20 structure to the interaction between Hg (6s6p) and the H 2 0 lone pair orbitals (detailed in ref 6), that a strengthening of the mercury-water bond occurs in the excited complex but not a weakening of the OH bond. Structure of the Complex. We have noted that the A constant is drastically reduced upon excitation, while the value of the B constant increases. The A constant corresponds to the rotation of the hydrogens about the Hg-O axis; thus we can have two interpretations for the A constant decrease. The first requires a drastic change in the water geometry: the opening of the H U H bond angle or the lengthening of the OH bond by ca. 0.5 A. This latter possibility is unlikely due to the absence of reactivity of the complex, which should dissociate upon collisions if it had a drastically weakened O H bond. In addition, the water geometry change should result in the appearance of 0-H stretch or H-0-H bend vibrational progressions in our spectra, which are not observed. In a second, more plausible hypothesis the reduction in the value of the A constant signals a conformational change upon excitation which we assign to a passage from a floppy ground-state Hg-H20 (where the water entity essentially undergoes free rotation) to a rigid, near-Ct, excited complex. We suspected that, in comparison with Ar-H20, the mercury-water ground-state complex should be a quasi-free rotor. The anisotropy in ground-state complexes is considered to be mainly due to the anisotropy of the induction forces.'* We have therefore calculated this potential due to the interaction of the water dipole and quadrupole with the induced dipole of ground-state mercury atoms, and it appears that the maximum anisotropy for the measured 3.6-A Hg-H20 equilibrium distance amounts to only 20 cm-', within the limit of this model. This value is small as compared to the water rotational level separations, but comparable to the deuterated water level separations. We thus consider the Hg-H20 ground-state complex as formed from mercury and a hindered rotor correlating to free water in its lowest ortho and para rotational states, Om and lo,, and to the rotational levels, K = 0 and 1, of a C, rigid bender. In Figure 4 these lowest levels of the complex are presumed to be separated by -24 cm-I. Furthermore, Endo2' has communicated to us that,
-
(I 8) See, for example: Klemperer, W. In Dynamics of polyatomic van der Wuuls complexes; Halberstadt, N., Janda, K., Eds.; Plenum: New York, 1990. (19) Duval, M. C.; Kassav, E. To be published. (20) Nesbitt, D. J.; Naaman, R. J . Chem. Phys. 1989, 91, 3801. (21) Endo, Y . Private communication, to be published. (22) Nelson, D. D., Jr.; Klemperer, W. J . Chem. Phys. 1987, 87, 139.
J. Phys. Chem. 1991,95, 9080-9085
9080
TABLE 11: Rotational Constants and Isotopic Shifts (Hg,*-Hg,) in cni’ 0: 9 IO 11 13 14 0.131 f 0.02 0.129 0.126 0.115 0.11 B: 8.5 8.1 8.9 8.8 8.9 f 0.1 AA 0.68 0.70 0.64 0.66 A,(202,204) 0.6 f 0.1
in his FTS microwave observations of the ground-state Hg-H20 complex, the first excited bending level correlating to K” = 0 lies above the one correlating to K” = 1 as is obvious from the correlation in Figure 4. Hence the supersonic expansion will prepare as the two levels with different nuclear spin statistics, the K” = 0 and 1 levels in Figure 4. In the optical excitation of the parallel A bands, the complex will retain the symmetry about the water C2axis: indeed the K ’ = 0 and K’ = 1 levels of the excited complex are separated by 15 cm-I, as deduced from the rotational spectrum in Figure 5. This 15-cm-I separation is close to the E constant of water. Hence, since this E constant corresponds to rotation about the b, Czaxis, the rigid excited Hg-HzO complex will be a prolate top with near-C, symmetry. This is further supported by the fact that the band head separation in Hg-D20 is twice as small (4.5 cm-I) as the separation (9 cm-I) in the Hg-H20 complex. We also observed, as shown in Table 11, that the band head separation starts to decrease slowly as the stretching quantum number increases, a sign that the anisotropy of the mercury-water potential decreases with increasing excitation. The rotational constants that we have deduced from the rotational contours are confirmed by ground-state rotational Fourier transform microwave spectroscopy: Endo et aLZ1find a similar
,-,
(23) We use the point group instead of c,(M)22 for the ground-state complex, undergoing major tunneling. This is for the sake of simplicity as, in our case, they are isomorphic.
value to ours (Table I) for the ground-state ( B + C)/2 constant for isotope 200 and indicate a value of the A constant close to 22 cm-I. We therefore conclude that there is a conformational change of the complex upon excitation, from a floppy ground state to a probable C, structure in the excited A state (although, as mentioned by Nesbitt,20an accurate geometry determination is not always possible). This C, excited geometry may also explain the complex structure observed within the B envelope, which is therefore partly due to the superposition of two electronic states of BI and Bzsymmetry (see ref 6). The conformational change is also the cause for the intense bending progression observed in Hg-DzO. On the other hand, the same progression should be observed in Hg-H20, even with a lesser intensity. This H / D isotope effect points out to a radiationless interconversion to the water ground state, channel 7, competing with the decay to 3P0. Such a process sensitive to the symmetry of intermolecular vibrations should be of great interest, especially if it leads to reactivity in the ground state; it will be subject to further investigation by time-resolved experiments. The structure of the complex is further confirmed by ab initio calculations of group IIB metal-water complexes,19 where the excited state of II configuration of the metal tends to adopt a C, conformation with a potential that is not too anisotropic. This conformation corresponds to the interaction of Hg (6s6p) with the water lone pairs, along the lines of the description in ref 6 . Acknowledgment. We are indebted to the European community for the support of their work by the Science program under contract SCI.Ol.IS.C.(JR). We also thank the GRECO “collisions r6actives” for financial support and Professor Endo for the communication of his results prior to publication. We are grateful . .H. . .Breckenridge, .. to w. A. Tramer, and c. Jouvet for many hearty and vivid discussions.
Photothermal Investigation of the Trlplet State of Cgo Masahide Terazima,* Noboru Hirota, Department of Chemistry, Faculty of Science, Kyoto University, Kyoto, 606, Japan
Hisanori Shinohara, and Yahachi Saitot Department of Chemistry for Materials, Mi’e University, Tsu,51 4, Japan (Received: April I O , 1991; In Final Form: June 4, 1991)
Photophysical properties of the triplet state of Cso(buckminsterfullerene) are investigated by photothermal techniques: thermal lens (TL), transient grating (TG), and photoacoustic (PA) methods. The quantum yield of singlet oxygen formation (aA) by the quenching of the triplet state of Cm is measured by the time-resolved TL method. The value of aAis nearly unity and independent of the excitation wavelength. The time dependence of the TL and TG signals does not show expected slow-rising components. This peculiar behavior can be explained in terms of a strong T-T absorption at the He-Ne laser wavelength. By using the PA method, the energy of the triplet state is determined to be 13 100 f 300 cm-I.
Introduction Since an intriguing proposal of the truncated-icosahedron structure of the Cm (buckminsterfullerene) cluster by Kroto et al. in 1985,’ a number of experimental and theoretical studies have been performed to confirm the proposed structure and to shed light on this novel form of the carbon cluster.2 Recently, moderately large-scale production and phase separation of C60and other fullerenes have been reported, respectively, by Krltschmer and co-worker3 and Kroto and c o - ~ o r k e r s .This ~ breakthrough virtually ensured very rapid development in understanding the ‘Department of Electrical Engineering, Mi’e University.
0022-3654/91/2095-9080$02.50/0
physical and chemical properties of Ca. This paper focuses on the electronic, particularly triplet, states of Cso. The nature of the photoexcited states of Cso has not been elucidated completely yet. in particular, the photophysical properties of the triplet state are hard to obtain because of the very weak radiative transition (absorption) due to spin-forbidden (1) Kroto, H. W.; Heath, J. R.; OBroen, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. (2) Weltner, W., Jr.; Van Zee, R. J. Chem. Rev. 1989, 89, 1713.
(3) Kritschmer, W.; Lamb, L. D.; Fostiropoulous, K.; Huffman, D. R. Nature 1990, 347, 354. (4) Taylor, R.; Hare, J. P.; Abdul-Sada, A. K.; Kroto, H. W. J . Chem. SOC.,Chem. Commun. 1990, 1423.
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